194 research outputs found
Bose-Einstein condensation of metastable helium: some experimental aspects
We describe our recent realization of BEC using metastable helium. All
detection is done with a micruchannel plate which detects the metastables or
ions coming from the trapped atom cloud. This discussion emphasizes some of the
diagnostic experiments which were necessary to quantitatively analyse our
results.Comment: 5 pages, 3 figure
Extension of the composite CBS-QB3 method to singlet diradical calculations
The composite CBS-QB3 method is widely used to obtain accurate energies of
molecules and radicals although its use in the case of singlet diradicals gives
rise to some difficulties. The problem is related to the parameterized
correction this method introduces to account for spin-contamination. We report
a new term specifically designed to describe singlet diradicals separated by at
least one CH2 unit. As a test case, we have computed the formation enthalpy of
a series of diradicals that includes hydrocarbons as well as systems involving
heteroatoms (nitrogen, oxygen). The resulting CBS-QB3 energies are very close
to experiment
The autoignition of cyclopentane and cyclohexane in a shock tube
Ignition delay times of cyclohexane-oxygen-argon and
cyclopentane-oxygen-argon mixtures have been measured in a shock tube, the
onset of ignition being detected by OH radical emission. Mixtures contained 0.5
or 1 % of hydrocarbon for equivalence ratios ranging from 0.5 to 2. Reflected
shock waves allowed temperatures from 1230 to 1800 K and pressures from 7.3 to
9.5 atm to be obtained. These measurements have shown that cyclopentane is much
less reactive than cyclohexane, as for a given temperature the observed
autoignition delay times were about ten times higher for the C5 compound
compared to the C6. Detailed mechanisms for the combustion of cyclohexane and
cyclopentane have been proposed to reproduce these results. The elementary
steps included in the kinetic models of the oxidation of cyclanes are close to
those proposed to describe the oxidation of acyclic alkanes and alkenes.
Consequently, it has been possible to obtain these models by using an improved
version of software EXGAS, a computer package developed to perform the
automatic generation of detailed kinetic models for the gas-phase oxidation and
combustion of linear and branched alkanes and alkenes. Nevertheless, the
modelling of the oxidation of cyclanes requires to consider new types of
generic reactions, and especially to define new correlations for the estimation
of the rate constants. Ab initio calculations have been used to better know
some of the rate constants used in the case of cyclopentane. The main reaction
pathways have been derived from flow rate and sensitivity analyses
Ionization rates in a Bose-Einstein condensate of metastable Helium
We have studied ionizing collisions in a BEC of He*. Measurements of the ion
production rate combined with measurements of the density and number of atoms
for the same sample allow us to estimate both the 2 and 3-body contributions to
this rate. A comparison with the decay of the number of condensed atoms in our
magnetic trap, in the presence of an rf-shield, indicates that ionizing
collisions are largely or wholly responsible for the loss. Quantum depletion
makes a substantial correction to the 3-body rate constant.Comment: 4 pages, 3 figure
Getting the elastic scattering length by observing inelastic collisions in ultracold metastable helium atoms
We report an experiment measuring simultaneously the temperatureand the flux
of ions produced by a cloud of triplet metastablehelium atoms at the
Bose-Einstein critical temperature. The onsetof condensation is revealed by a
sharp increase of the ion fluxduring evaporative cooling. Combining our
measurements withprevious measurements of ionization in a pure BEC,we extract
an improved value of the scattering length nm. The analysis
includes corrections takinginto accountthe effect of atomic interactions on the
criticaltemperature, and thus an independent measurement of the
scatteringlength would allow a new test of these calculations
Thermal decomposition of norbornane (bicyclo[2.2.1]heptane) dissolved in benzene. Experimental study and mechanism investigation
The thermal decomposition of norbornane (dissolved in benzene) has been
studied in a jet stirred reactor at temperatures between 873 and 973 K, at
residence times ranging from 1 to 4 s and at atmospheric pressure, leading to
conversions from 0.04 to 22.6%. 25 reaction products were identified and
quantified by gas chromatography, amongst which the main ones are hydrogen,
ethylene and 1,3-cyclopentadiene. A mechanism investigation of the thermal
decomposition of the norbornane - benzene binary mixture has been performed.
Reactions involved in the mechanism have been reviewed: unimolecular
initiations 1 by C-C bond scission of norbornane, fate of the generated
diradicals, reactions of transfer and propagation of norbornyl radicals,
reactions of benzene and cross-coupling reactions
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